An intriguing question in cell biology relates to the mechanism(s) by which proteases activate cells. In recent years, a subfamily of G protein-coupled receptors capable of mediating cellular signaling in response to proteases has been identified (T. K. H. Vu et al, Cell 64:1057–68, 1991; U. B. Rasmussen et al. FEBS Lett. 288:123–28, 1991; S. Nystedt et al., Proc. Natl. Acad. Sci. USA 91:9208–12, 1994; H. Ishihara et al., Nature 353:674–77, 1997). Members of this unique G protein-coupled receptor family include protease-activated receptors PAR1, PAR2 and PAR3. These receptors are characterized by a tethered peptide ligand at the extracellular amino terminus that is generated by minor proteolysis.
The first identified member of this family was the thrombin receptor presently designated protease-activated receptor 1 (PAR1). Thrombin cleaves an amino-terminal extracellular extension of PAR1 to create a new amino terminus that functions as a tethered ligand and intramolecularly activates the receptor (T. K. H. Vu et al, Cell 64:1057–68, 1991). PAR2 mediates signaling following minor proteolysis by trypsin or tryptase, but not thrombin (S. Nystedt et al., Proc. Natl. Acad. Sci. USA 91:9208–12, 1994). Knockout of the gene coding for PAR1 provided definitive evidence for a second thrombin receptor in mouse platelets and for tissue-specific roles for different thrombin receptors (A. Connolly et al., Nature 381:516–19, 1996). PAR3 was identified recently as a second thrombin receptor mediates phophatidyl inositol 4,5 diphosphate hydrolysis, and was found to be expressed in a variety of tissues (H. Ishihara et al., Nature 353:674–77, 1997). Many other proteases (such as factor VIIa, factor Xa, factor XIIa, protein C, neutrophil cathepsin G, mast cell tryptase, and plasmin) display cellular effects. Therefore, additional members of the PAR family are expected to exist (S. R. Coughlin, Proc. Natl. Acad. Sci. USA 91:9200–02, 1994; M. Molino et al., J. Biol. Chem. 272:11133–41, 1997).
The present invention provides an additional member of the PAR family, a novel human protease-activated receptor designated PAR4 (alternatively designated ZCHEMR2). The PAR4 polypeptide is an appropriate target for drug screening, and has other uses that should be apparent to those skilled in the art from the teachings herein.